Prime cuts.

Not getting enough good, clean regrind? You need to know more about picking the right granulator.

Ten or 15 years ago, scrap granulating wasn't considered high technology. Even today some processors persist in thinking that size reduction requires no special considerations as long as the machine is big enough to take whatever scrap is thrown into it. But overlooking the importance of a granulator to your overall production efficiency and product quality could prove detrimental to both. If you're putting scrap back into your process, you want to pay close attention to how it's treated during size reduction.

You could experience as much as a 20-40% improvement in product quality and regrind output rate if you sharpen your approach to granulator selection. You have to start with a clear understanding of the material to be ground - its composition (hard, soft, heat-sensitive, or stable), physical form (parts, runners, flash, rolls, or "haystacks" of film), size, wall thickness, and temperature. You also have to consider how and at what rate the scrap will be fed to the granulator and how the regrind will be removed afterward. All these factors help determine the most appropriate granulating machine size, feed-throat and cutting-chamber design, rotor and knife configuration, rotor speed, drive horsepower, screen size, and granulate take-away system.

Ideally, a granulator should be specified to process a particular feedstock under particular conditions. Don't expect to use it as a general-purpose machine to process all feedstocks under all operating conditions. Realistically, you will want some degree of versatility, but that should be built into your specifications and may require some compromises in performance.

GET THE FINES OUT

The first goal in granulating is to produce uniform granulate in a size close to that of the virgin resin and with a minimum of fines. The closer the bulk density of regrind is to that of virgin, the less trouble you will have in feeding the two in a consistent ratio. Granule size affects bulk density more than does the specific gravity of the material. What's more, a cleaner recyclate with fewer fines will help keep hopper-loader screens cleaner and prevent regrind from bridging in vacuum receivers.

You must answer six principal questions about your application before you and your equipment supplier can begin to specify the granulator you need (see sidebar). The correct granulator selection matches the rotor and knife design, knife clearances, rotor speed, and screen sizing to the infeed material's properties and physical form. Lack of attention to any of these factors can easily result in either excessive fines or "longs" (oversized pieces).

MATERIAL MATTERS

Many granulator design choices depend on the material to be cut. Soft, flexible, energy-absorbing materials generally produce the most uniform particles. Rigid, friable materials tend to fracture upon impact from the rotating knives and will generate less uniform particle sizes. Engineering materials that are filled with glass or minerals are often brittle and susceptible to impact fracturing.

Size, shape, and thickness of the infeed material are also critical. Thin parts must be cut differently from thick purgings or heavy-walled pipe. Bulky but lightweight, thin-walled parts like blow molded bottles also present distinct challenges to efficient granulating.

Don't neglect to consider the temperature of the infeed material. Warm material is typically less brittle and easier to cut (think of a knife through butter). Examples are when a granulator receives purged or start-up material directly from the extruder, blow molder, injection press or thermoformer. Torque requirements may be significantly lower than for cutting a similar volume of cold plastic. However, the material shouldn't be too warm, or else it will smear on contact with the granulator knives and clog screen holes as well as reduce output.

FEEDING THE BEAST

Material can be introduced into the granulator by hand, conveyor, robot, gravity chute, and ram, roll, or auger feeder. The shape of the feedstock and other factors will determine your choice.

A feed hopper must be evaluated for operator safety. Most safety devices in today's granulators involve electrical interlocks that prevent access to the feed hopper (and the screen) until the rotors have stopped turning. Older granulators may not have such built-in safeties and are inherently dangerous to the people who work with them.

There are two basic cutting-chamber designs. The popular tangential-feed type positions the rotor at an offset from the feed opening so that the feedstock is directed into the downward cut of the rotating knives at a tangent to the cutting circle. This chamber style is preferred for granulating bottles, injection molded tv cabinets, or other bulky, thin-walled parts. The tangential cutting chamber provides a large "bite" radius, which is the most efficient way to achieve both high throughput and clean regrind with such parts. The more tangential the infeed, the greater the "bite" of the knives and the less likelihood of tossing the part around before it is thoroughly chopped.

On the other hand, a straight drop-through feed hopper and cutting chamber - which feed material perpendicular to the cutting circle - are recommended for thick-walled parts. This design imparts a "nibbling" type of bite that is less likely to smash the part into fragments or take too big a bite and thereby stall the rotor.

A number of granulator manufacturers make hybrid models that offer a degree of flexibility by combining the benefits of both the tangential-feed and straight-through designs. An extra bed knife in the right position can reduce the size of the bite.

A separate but important consideration is whether material will be fed steadily (such as with roll or batch feeding) or intermittently (e.g., a runner system after each press cycle). Batch feeding can improve granulate quality because material is ground under a "head" of pressure. What's more, having a head of material in the cutting chamber dampens noise by keeping scrap from bouncing around the machine's innards.

ONE SIZE DOESN'T FIT ALL

Pick too small a machine and you could be left with no option but to slice up parts manually with a bandsaw to fit the granulator - a labor-intensive and dangerous operation.

So when selecting a granulator, it is best to over-specify size and horsepower. The feed hopper should be sized to receive the largest part being granulated without precutting. Trying to save money by specifying a smaller granulator whose cutting chamber is too close in size to the scrap being fed often results in feeding problems and hang-ups that consume operator time and attention. It will definitely cost you more money in the long run if you have to take steps to orient parts a certain way so that they will fit into the cutting chamber.

MAKING THE CUT

Inside the machine, there are three components whose selection directly affects the quality of regrind produced. These are the rotor, the rotating and stationary knives, and the screen. In addition, operating factors like knife speed and clearances between the knives and the screen have to be addressed.

The three main types of rotors are open, closed (or solid), and staggered. Open rotors provide unrestricted air flow through the cutting chamber and so are more often selected for granulating heat-sensitive resins or feedstocks that are still quite warm from processing. Open-rotor designs also most readily provide for taking a big "bite" of bulky thin-walled parts.

Closed or solid rotors have no open spaces between the rotating knives and the center of the shaft, providing a much stronger knife-mounting arrangement and adding inertia to cut through the thickest feedstocks, such as large purgings, especially cold ones.

Staggered rotors are offered in closed or semi-closed configurations and are excellent for cutting most heavy, thick-walled parts. A helical, segmented rotor combines the advantages of open and closed rotors, providing more cuts per revolution and requiring less horsepower for comparable throughput.

In heavy-duty applications, fly-wheel-type pulleys increase rotor inertia and are often a practical way to achieve more cutting force without adding horsepower. More or bigger motors not only cost more up front than flywheels, but they also eat up more electricity in use.

A "slant-knife" arrangement that produces a scissors cutting action is much preferred nowadays over the traditional straight-cut knife mounting because of its efficiency in producing high throughputs with less horsepower, less noise, and reduced fines and dust. The scissors cut is achieved by offsetting the rotor and/or bed knives at a small angle. This produces a slicing or shearing action with a smaller area of impact than is produced by a straight-knife design, which chops along the entire knife length at one blow.

Tip angle may have an impact on cutting force. Low angles have blunter knife tips that are suitable for brittle materials. High knife angles accompanied by a sharp knife edge are used to cut softer and tougher materials.

While many granulator designs offer only two stationary or bed knives, a third or fourth bed knife on some sizes of granulators can significantly increase throughput by increasing the number of cuts per revolution. Increasing the number of blades on the rotor can have a similar effect.

The clearance between rotating and bed knives is crucial to size-reduction performance. The smaller the clearance, the more efficient and cleaner the cut will be. Smaller clearances are also more important when grinding softer, more ductile polymers. Adjustable bed knives are valuable because in most cases resharpening a rotor knife reduces its size and thereby shortens the cutting circle. In order to maintain an optimum knife clearance - typically 0.003-0.005 in. - the bed knife must be adjusted to move its blade edge closer to the shortened rotor blade. While that solves one problem, another remains in the greater gap between the rotating knife tips and the screen. Oversized granulate pieces can plug up the screen holes, lowering output and producing more fines and dust because less open screen area is available.

Adjustable rotor knives - a fairly new feature - allow you to keep the clearances as small as possible and to [TABULAR TABLE OMITTED] keep them constant, even after sharpening. A helical, staggered rotor will also maintain a uniform clearance between rotor and bed knives after sharpening since the latter does not affect the outside diameter. Adjustability ensures maximum throughputs, especially when processing film, sheet, or other thin feedstocks.

Knife sharpness is also very important to both granulate quality and the granulator's cutting performance, so a periodic maintenance program should be established to check knife sharpness and wear regularly. Frequency of knife sharpening will depend on the abrasiveness of the material being cut - filled and reinforced compounds are the worst - and on the steal alloy composition of the blades. Knife steels are generally designed for either toughness or wear resistance. For soft, non-abrasive materials, knives made of chrome-vanadium steal (CVS) are tough and resistant to chipping but wear faster than other tool steels. D2 steel, the most common choice for granulator knives, is harder and provides more wear resistance but is more susceptible to chipping, which can be a problem if tramp metal is present. A more expensive option is M42 tool steal, whose wear resistance is five times that of the D2 steel, though it is just as brittle. If tramp metal is a potential problem, the less expensive D2 steel may be the prudent choice.

SPIN SLOWER TO RUN FASTER

To increase your output of uniform granulate, the solution is a granulator designed for a lower rotor speed. Lower knife-tip speed gives each granule more of a chance to pass though the screen. With a high-speed rotor, the particles are more susceptible to being carried around the machine or frayed into fines by repeated cuts. With abrasive materials, slower rotor speeds reduce knife and cutting-chamber wear. Slower speeds also can reduce noise.

Knife-tip speed is partly a function of rotor diameter - hence, the same rotor rpm on two machines with different sized cutting circles will produce different results. Rotor speeds are typically fixed, but an optional variable-speed device can be attached to your machine to provide some flexibility. Unfortunately, there are no rules for determining what rotor speed is optimal. Most granulator suppliers offer application laboratories where you can determine the optimum rotor speed for your application.

When running at slower speeds, smaller screen holes and a thinner screen will produce more uniform particle sizes and highest throughput. Conversely, at higher rotor speeds, larger screen holes are necessary to maintain throughput, but larger holes can increase the range of particle sizes produced, particularly when granulating brittle materials. Though heavily dependent on material composition and machine size, screen hole diameters are typically 3/16 in. for rotor speeds under 200 rpm, 5/16 in. for speeds of 300-400 rpm, and 3/8 in. for speeds above 400 rpm. A reversible screen will greatly increase the screen's useful life.

RETURN TO THE PROCESS

Tired of unloading granulator bins by hand? There are good reasons for you to consider automatic evacuation and conveying right back to the process hopper for computer-controlled ratio blending with virgin resin. Besides saving manual handling costs and storage space, an automatic, closed-loop reprocessing system lessens the chance for contaminating the regrind or inadvertently adding it to the wrong machine hopper. Using a blower or vacuum loader also helps draw material through the screen quickly and prevents granulate from accumulating on top of the screen. The result is improved throughput and reduced fines and dust.

WHERE WILL YOU GRIND IT?

There are three basic types of granulators - central, beside-the-press, and under-the press - each offering its own benefits and disadvantages. A central unit can handle large bulky parts as well as large quantities of smaller ones and commonly does granulating duty for a number of lines or presses in the plant. It isolates the noise in a separate area. Conair denotes any machine with at least an 18 x 24 in. feed opening as a central granulator, while anything smaller can be situated beside or under the press (or extruder or thermoformer).

The central granulator is typically used as an off-line device located in its own room to isolate the noise. Because of its role as a do-everything size reducer, it becomes a potential agent of contamination if not cleaned properly. Since central granulators are necessarily sized to handle the largest product being fed, cleaning between material changes is a bigger job. Storage space must also be reserved for the inventory of scrap waiting to be ground and the regrind produced. There's also the necessity of transporting the scrap from where it is produced to the granulator and transporting the regrind back to processing lines.

Under-the-press granulators are a decentralized alternative. Scrap can be gravity-fed directly from a processing machine without conveying or manual handling. Auger granulators, once popular for under-the-press grinding of sprues and runners, are becoming less popular today because of their inherent safety risks. Although designed and dearly labeled for under-machine use exclusively, operators frequently use them beside the press. This exposes the open auger and presents a risk of injury if anyone reaches into the auger to clear a jam while the unit is running. Though feed hoppers can be built over the auger opening to protect against accidents, hoppers are usually still open to foreign objects falling into the auger and cutting chamber. Auger granulators can be interlocked so they will operate only under the press, but interlocks too often are bypassed or over-ridden.

Upright beside-the-press granulators and robotic sprue feeding are by far the safest and preferred methods. Beside-the-press grinding should be considered to minimize scrap handling, eliminate extra storage space, and reduce contamination by dedicating a specific granulator to a specific material. Generally, these granulators will be smaller and easier to clean. And they offer portability from machine to machine. The disadvantage is that this machine type can bring more noise to your plant floor if not properly sound-dampened. An especially good case can be made for beside-the-press grinding of brittle materials because it permits grinding scrap while it is still warm from processing, making it easier to cut and less likely to shatter into fines. Noise levels may also be reduced and lower horsepower may be needed, saving energy.

Ask Yourself These Questions Before You Buy a Granulator

1 WHAT DO YOU WANT TO GRIND?

* Consider: Type of part or sprue/runner system, type of material (hard/soft, brittle/tough, filled/unfilled), and temperature of parts being granulated.

* Determines: Rotor design, horsepower, sound attenuation, and means of regrind removal.

2 WHAT SIZE IS THE PART OR SCRAP?

* Consider: Physical size, shape, and wall thickness of the parts or runners.

* Determines: Feed-hopper and cutting-chamber size and configuration (tangential or horizontal).

3 HOW MUCH DO YOU WANT TO GRANULATE?

* Consider: Desired throughput rate (more important for central grinding, but can be significant when one beside-the-press granulator is to serve several machines processing the same material).

* Determines: Horsepower, rotor design, number of rotating and bed knives, screen size, rotor speed, and means of regrind removal.

4 HOW WILL YOU FEED THE GRANULATOR?

* Consider: Manual batch feeding or metered feed by hand, robot, or conveyor.

* Determines: Feed-hopper design.

5 WHAT WILL YOU DO WITH THE REGRIND?

* Consider: Particle-size requirement and whether it will be reprocessed alone or proportioned with virgin.

* Determines: Quality (uniformity) required, screen size, rotor speed, and evacuation method (e.g., proportional feeding to a hopper loader, blower to storage).

6 WHAT ARE YOUR SPECIAL NEEDS?

* Consider: Location of regrinding, diversity of grinding requirements, cleanliness requirements, control requirements, and safety and noise requirements.

* Determines: Central or beside-the-press installation, special controls, safeties, alarms, level indicators, sound attenuation, portability, and many other options.